There is technology of putting a value in an empty pixel from peripheral pixels as a method of interpolating a pixel value. For example, bilinear interpolation and bicubic interpolation are typical examples; this may easily cause jaggies (uneven edge which is not smooth) in an interpolation result.
Although there also is a method of interpolating by using a low-pass filter with a low pass frequency band, this does not cause the jaggies but the image is easily blurred.
Therefore, technology of calculating similarity between positions symmetrical about a current position by block matching, determining whether they belong to the same pattern as that of the current pixel, and obtaining an interpolated value by an average value thereof, thereby enabling image enlargement in one direction is suggested as a method of using the block matching (refer to Patent Document 1).
Also, technology of reducing the jaggies by adding pixel values in the positions symmetrical about the current position in the same manner is suggested (refer to Patent Document 2). In this method corresponding to many angles, a jaggy opposite phase which reduces the jaggies in principle is searched and the pixel values in one line above and below the current pixel on an interpolated image are utilized.